Framing structure for digital broadcasting and interactive services

ABSTRACT

An approach is provided for supporting frame synchronization in a digital broadcast and interactive system. A transmitter includes an encoder that outputs a Low Density Parity Check (LDPC) codeword. The transmitter also includes a framing module generates a LDPC coded frame in response to the LDPC codeword, and appends a physical layer signaling field to the LDPC codeword for specifying modulation and coding information associated with the LDPC coded frame. The physical layer signaling field is encoded with a Forward Error Correction (FEC) code and has an embedded framing structure to assist with frame synchronization. The above arrangement is particularly suited to a digital satellite broadcast system.

RELATED APPLICATIONS

[0001] This application claims the benefit of the earlier filing dateunder 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No.60/478,376 filed Jun. 13, 2003, titled “Framing and Synchronization forDigital Satellite Broadcasting and Interactive Services,” U.S.Provisional Application Ser. No. 60/482,111 filed Jun. 24, 2003, titled“Framing and Synchronization for Digital Satellite Broadcasting andInteractive Services,” and U.S. Provisional Application Ser. No.60/482,117 filed Jun. 13, 2003, titled “Framing Structure andAcquisition Method for Rapid Synchronization”; the entireties of whichare incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to communication systems, and moreparticularly to digital communication systems.

BACKGROUND OF THE INVENTION

[0003] Broadcast systems have embraced the demand for high qualitytransmissions made possible by digital technology. The digitalrevolution has transformed the delivery of broadband services, includingaudio and video programming as well as data transmission. Satellitecommunication systems have emerged as a viable solution for supportingsuch broadband services. As such, power and bandwidth efficientmodulation and coding are highly desirable for satellite communicationssystems to provide reliable communication across noisy communicationchannels. In broadcast applications supported by such systems, rapidframe synchronization in low signal-to-noise (SNR) environments isnecessary to avoid negatively impacting user experience, as well asutilizing system resources efficiently.

[0004] Traditionally, frame synchronization has not been an area ofmajor concern for conventional broadcast and/or continuous transmissionsystems employing convolutional code, largely because decoding can beperformed prior to frame synchronization. Consequently, the postdecoding frame synchronization can benefit from the coding gain offeredby the error correction codes. For instance, the Digital VideoBroadcasting via Satellite (DVB-S) standard has been widely adoptedworldwide to provide, for example, digital satellite televisionprogramming. Traditional DVB compliant systems employ fixed modulationand coding schemes. At present, such DVB compliant systems utilizeQuadrature Phase Shift Keying (QPSK) modulation and concatenatedconvolutional code and Reed-Solomon channel coding. Given the fact thatmodulation and coding schemes are fixed, and the fact that thecontinuous transmission nature of broadcasting or unicasting, a simpleframing structure can be utilized for these applications. In actuality,the only framing overhead is a Synchronization (“SYNC”) byte attached toa MPEG 2 (Moving Pictures Experts Group-2) frame. The SYNC byte istreated the same as other data by the convolutional code and theReed-Solomon encoder. At the receiving end, the data corrupted by thecommunication media are first recovered by the convolutional code. Theconvolutional code can function without the knowledge of the framingstructure. The output of the convolutional code is of high fidelity,typically at bit error rate below 1×10⁻⁵. With the high fidelity output,simple data matching with the SYNC byte is able to identify the startingpoint of the MPEG frame. Therefore, the transmitted data can be properlyreassembled to deliver to the next layer.

[0005] However, with block coded systems, frame synchronization istypically achieved before decoding. This is required particularly whenthe receiver has to determine which modulation and coding is used amonga vast amount of potential combinations of modulation and codingschemes. Modern error correction coding, such as low density paritycheck (LDPC) codes, operates at extremely low signal to noise ratios.This implies that such frame synchronization needs to be achieved at thesame low signal-to-noise ratios (S/N or SNR). Furthermore, framesynchronization in such systems extends beyond determining the beginningand ending points of a frame, to determining the modulation and codingscheme employed in the frame.

[0006] In view of the foregoing, the conventional approaches to framesynchronization do not operate well in that the requirements of highfidelity outputs, for example, can no longer be guaranteed.

[0007] Consequently, other approaches have been developed, but requireincurring significant overhead (i.e., reduction in throughput) andreceiver complexity. For example, one approach suggests using a forwarderror correction coding, such as a Bose Chaudhuri Hocquenghem (BCH)code, to protect the framing information within the frame structure. Atthe receiving end, the receiver searches for the unique word first bycorrelation. Once the unique word is detected, the BCH coded framinginformation is decoded coherently by a maximum likelihood correlationdecoding. A drawback of this technique is that the unique word has to belarge (i.e., high overhead). Another drawback is that true maximumlikelihood decoding of the BCH code is quite complex.

[0008] Therefore, there is a need for a frame synchronization mechanismthat provides rapid acquisition without incurring large overhead costs.There is also a need for a frame synchronization approach that is simpleto implement. There is also a need to provide a synchronizationtechnique that is flexible as to provide coding and modulationindependence.

SUMMARY OF THE INVENTION

[0009] These and other needs are addressed by the present invention,wherein an approach is provided to support frame synchronization in adigital broadcast system utilizing Low Density Parity Check (LDPC)codes. A framing module includes a constellation mapper for mapping acodeword (e.g., generated by a Reed-Muller encoder) specifying framinginformation of a frame according to a signal constellation to output adata stream. The data stream is split into two data streams. One of thedata stream is modified to transmit an additional bit (each of which ismultiplied by a constant depending on the information bit transmitted,in the binary domain, this implies that the data stream includes eithera duplicate of the original data stream or a binary complement of theoriginal data stream). The two data streams are then combined to formthe physical layer signaling code, which is appended to an LDPC codedframe. This approach embeds a framing structure that can assist withsynchronization. On the receiving side, a relatively simple framedetector can be used to locate the unique word and physical layersignaling code based on the embedded framing structure of the physicallayer signaling code. This information is then supplied to a peak searchdetection process, which searches for a peak value within a searchwindow, and designates this peak value as a candidate. The search windowlength can be set according to the modulation scheme employed, if known;otherwise, a default length is used. The peak search can be conductedover multiple search windows, resulting in other candidates. After eachsearch, the candidate is verified by deriving the location of the nextpeak from the particular candidate. The above arrangement advantageouslyprovides rapid and reliable frame acquisition without additionaloverhead.

[0010] According to one aspect of an embodiment of the presentinvention, a method for supporting frame synchronization in a digitalcommunication system is disclosed. The method includes mapping acodeword specifying framing information of a frame according to a signalconstellation to output a data stream. Additionally, the method includesduplicating and demultiplexing the data stream into a first data streamand a second data stream. The method also includes modifying the firstdata stream according to a predetermined operation; and multiplexing themodified first data stream with the second data stream. Further, themethod includes outputting a physical layer header corresponding to theframe based on the multiplexed data streams.

[0011] According to another aspect of an embodiment of the presentinvention, an apparatus for supporting frame synchronization in adigital communication system is disclosed. The apparatus includes aconstellation mapper configured to map a codeword specifying framinginformation of a frame according to a signal constellation to output adata stream, wherein the data stream is demultiplexed into a first datastream and a second data stream. The apparatus also includes amultiplier coupled to the constellation mapper and configured to modifythe first data stream. Further, the apparatus includes a multiplexerconfigured to combine the modified first data stream with the seconddata stream, wherein a physical layer header corresponding to the frameis output based the multiplexed data streams.

[0012] According to another aspect of an embodiment of the presentinvention, a method of supporting frame synchronization in a digitalbroadcast system is disclosed. The method includes encoding framinginformation of a frame by a forward error correction code to outputencoded bits. The method also includes repeating each of the encodedbits. The method further includes modifying the repeated bits accordingto a predetermined operation to transmit additional framing information.

[0013] According to another aspect of an embodiment of the presentinvention, a method for detecting the start of a frame is disclosed. Themethod includes receiving a data stream corresponding to a broadcastsignal. The data stream includes a unique word and a physical layerheader specifying modulation and coding information of the broadcastsignal. The method also includes differentiating the data stream;multiplying the differentiated data stream with a predeterminedmultiplier; summing outputs of the multiplication; adding the summedoutputs to yield a plurality of added values; and subtracting the summedoutputs to yield a plurality of subtracted values. The method furtherincludes determining a maximum value among absolute values of the addedvalues and the subtracted values.

[0014] According to another aspect of an embodiment of the presentinvention, a device for detecting the start of a frame is disclosed. Thedevice includes means for receiving a data stream corresponding to abroadcast signal. The data stream includes a unique word and a physicallayer header specifying modulation and coding information of thebroadcast signal. The device also includes means for differentiating thereceived data stream; means for multiplying the differentiated datastream with a predetermined multiplier; means for summing outputs of themultiplication; means for adding the summed outputs to yield a pluralityof added values; means for subtracting the summed outputs to yield aplurality of subtracted values; and means for determining a maximumvalue among absolute values of the added values and the subtractedvalues.

[0015] According to another aspect of an embodiment of the presentinvention, a method for recovering framing information of a frametransmitted over in a digital communication system is disclosed. Themethod includes descrambling a physical layer signaling code of theframe. The physical layer signal code is encoded according to a firstorder Reed-Muller code and interleaved. The method also includesdecoding the physical layer signaling code to derive coding rate,modulation format, and pilot structure of the frame.

[0016] According to yet another aspect of an embodiment of the presentinvention, a method for supporting frame synchronization in a digitalcommunication system is disclosed. The method includes setting a searchwindow length; and determining location of a peak within a frame overthe search window length. The frame includes a unique word, a codeword,and a coded segment, wherein the codeword specifies framing informationof the coded segment. The method also includes designating the peaklocation as a candidate; and verifying the candidate. Further, themethod includes declaring acquisition of the frame if the candidate isverified.

[0017] According to yet another aspect of an embodiment of the presentinvention, a transmitter includes an encoder configured to output a LowDensity Parity Check (LDPC) codeword. The transmitter also includes aframing module configured to generate a LDPC coded frame in response tothe LDPC codeword, and to append a physical layer signaling field to theLDPC codeword for specifying modulation and coding informationassociated with the LDPC coded frame. The physical layer signaling fieldis encoded with a Forward Error Correction (FEC) code and has anembedded framing structure to assist with frame synchronization.

[0018] Still other aspects, features, and advantages of the presentinvention are readily apparent from the following detailed description,simply by illustrating a number of particular embodiments andimplementations, including the best mode contemplated for carrying outthe present invention. The present invention is also capable of otherand different embodiments, and its several details can be modified invarious obvious respects, all without departing from the spirit andscope of the present invention. Accordingly, the drawing and descriptionare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

[0020]FIG. 1 is a diagram of a digital broadcast system configured toutilize Low Density Parity Check (LDPC) codes, according to anembodiment of the present invention;

[0021]FIG. 2 is a diagram of an exemplary transmitter employed in thedigital transmission facility of the system of FIG. 1;

[0022]FIG. 3 is a diagram of an exemplary digital modem in the system ofFIG. 1;

[0023]FIG. 4 is a diagram of an exemplary frame structure, in accordancewith an embodiment of the present invention;

[0024]FIG. 5 is a diagram of a physical layer signaling informationfield generator utilizing a Binary Phase Shift Keying (BPSK)constellation, in accordance with an embodiment of the presentinvention;

[0025]FIG. 6 is a flowchart of the operation of a physical layersignaling information field generator, in accordance with an embodimentof the present invention;

[0026]FIG. 7 is a flowchart of a frame detection process, in accordancewith an embodiment of the present invention;

[0027]FIG. 8 is a diagram of a detector utilizing physical layersignaling information, in accordance with an embodiment of the presentinvention;

[0028]FIG. 9 is a diagram of a differential detector, in accordance withan embodiment of the present invention;

[0029]FIG. 10 is a diagram of a peak search detection scheme, inaccordance with an embodiment of the present invention;

[0030]FIG. 11 is a flowchart of a peak search process, in accordancewith an embodiment of the present invention;

[0031]FIG. 12 is a diagram of the detector of FIG. 8 modified forbuffering and accumulation, in accordance with an embodiment of thepresent invention; and

[0032]FIG. 13 is a diagram of a computer system that can perform thevarious processes associated with frame synchronization, in accordancewith embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] An apparatus, method, and software for efficiently providingframe synchronization in a digital broadcast system are described. Inthe following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It is apparent, however, to oneskilled in the art that the present invention may be practiced withoutthese specific details or with an equivalent arrangement. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

[0034]FIG. 1 is a diagram of a digital broadcast system configured toutilize Low Density Parity Check (LDPC) codes, according to anembodiment of the present invention. The digital communications system100 includes a digital transmission facility 101 that generates signalwaveforms for broadcast across a communication channel 103 to one ormore digital modems 105. According to one embodiment of the presentinvention, the communication system 100 is a satellite communicationsystem that supports, for example, audio and video broadcast services aswell as interactive services. Interactive services include, for example,electronic programming guides (EPGs), high-speed internet access,interactive advertising, telephony, and email services. Theseinteractive services can also encompass such television services as PayPer View, TV Commerce, Video On Demand, Near Video On Demand and AudioOn Demand services. In this environment, the modems 105 are satellitemodems.

[0035] These modems 105 achieve carrier synchronization by examining thepreambles and/or unique words (UW) that are embedded in broadcast dataframe structures (shown in FIG. 4), thereby avoiding the use ofadditional overhead specifically designated for training purposes. Thedigital modems 105 are more fully described below with respect to FIG.3.

[0036] In this discrete communications system 100, the transmissionfacility 101 produces a discrete set of possible messages representingmedia content (e.g., audio, video, textual information, data, etc.);each of the possible messages has a corresponding signal waveform. Thesesignal waveforms are attenuated, or otherwise altered, by communicationschannel 103. To combat the noise channel 103, the transmission facility101 utilizes LDPC codes.

[0037] The LDPC codes that are generated by the transmission facility101 enable high speed implementation without incurring any performanceloss. These structured LDPC codes output from the transmission facility101 avoid assignment of a small number of check nodes to the bit nodesalready vulnerable to channel errors by virtue of the modulation scheme(e.g., 8PSK). Such LDPC codes have a parallelizable decoding process(unlike turbo codes), which advantageously involves simple operationssuch as addition, comparison and table look-up. Moreover, carefullydesigned LDPC codes do not exhibit any sign of error floor.

[0038] According to one embodiment of the present invention, thetransmission facility 101 generates, using a relatively simple encodingtechnique as explained below in FIG. 2, LDPC codes based on parity checkmatrices (which facilitate efficient memory access during decoding) tocommunicate with the satellite modem 105.

[0039]FIG. 2 is a diagram of an exemplary transmitter employed in thedigital transmission facility of the system of FIG. 1. A transmitter 200is utilized in the facility 101 to support, for instance, digitalbroadcasting and interactive services. An information source 201provides information bits to an LDPC encoder 203, which outputs codedstream of higher redundancy suitable for error correction processing atthe receiver 105. The coded stream is supplied to a framing module 204to generate a transmission frame, which can include a unique word (UW)and a physical layer signaling header for conveying framing informationof the LDPC coded frame.

[0040] LDPC codes require, in general, specifying the generatormatrices. The LDPC encoder 203 uses a simple encoding technique thatmakes use of only the parity check matrix by imposing structure onto theparity check matrix. Specifically, a restriction is placed on the paritycheck matrix by constraining certain portion of the matrix to betriangular. Such a restriction results in negligible performance loss,and therefore, constitutes an attractive trade-off. The construction ofsuch a parity check matrix is described more fully described in aco-pending patent application filed Jul. 3, 2003, and entitled, “Methodand System for Providing Low Density Parity Check (LDPC) Encoding”(Attorney Docket No. PD-203016; Ser. No. 10/613,823); the entirety ofwhich is incorporated herein by reference.

[0041] Modulator 205 maps the transmission frame from the framing module204 to signal waveforms that are transmitted to a transmit antenna 207,which emits these waveforms over the communication channel 103.Accordingly, the encoded messages are modulated and distributed to atransmit antenna 207. The transmissions from the transmit antenna 207propagate to a digital modem, as discussed below. In the case of asatellite communication system, the transmitted signals from the antenna207 are relayed via a satellite.

[0042]FIG. 3 is a diagram of an exemplary digital modem in the system ofFIG. 1. The digital modem 300, as a modulator/demodulator, supports bothtransmission and reception of signals from the transmitter 200.According to one embodiment of the present invention, the modem 30 has aframe synchronization module 301 that provides frame acquisition of LDPCencoded signals received from antenna 303. A demodulator 305 performsdemodulation of received signals output from the carrier synchronizationmodule 301. After demodulation, the signals are forwarded to a LDPCdecoder 307, which attempts to reconstruct the original source messages(i.e., information bits).

[0043] On the transmission side, the modem 305 utilizes a LDPC encoder309 to encode input signals. The encoded signals are then modulated by amodulator 311, which can employ a variety of modulation schemes—e.g.,BPSK (Binary Phase Shift Keying), QPSK, 8PSK, 16 APSK (Amplitude PhaseShift Keying), or other higher order modulation.

[0044] Alternatively, in a strictly broadcast application, the modulator311 may not be required, as an end user would not have a need totransmit back to the broadcast network. The modulator 205, as part ofthe transmitter 200, can reside within a broadcast center, while thedemodulator 305 can be deployed in the end user's home. Under thisconfiguration, the end user will have a receive-only terminal.

[0045]FIG. 4 shows a diagram of an exemplary frame structure, inaccordance with an embodiment of the present invention. By way ofexample, a frame structure 400 is designed to support digitalbroadcasting system of FIG. 1. The system 100, as noted, can be deployedas a satellite communication system. As such, the frame structure 400 iscompliant with Digital Video Broadcasting (DVB)-S2 standard, whichsupports, for example, satellite broadcasting and interactive services.

[0046] Given the advancement in power, satellite systems can supportefficient and dynamic coding and modulation schemes, such as LDPC codingscheme and higher order modulation. By dynamically specifying coding andmodulation schemes, the transmission can be adapted to the environment(e.g., rainy conditions, clear skies, etc.) to optimize throughput.However, the dynamic coding and modulation schemes impose significantconstraints and requirements on the framing structure. Because themodulation scheme is dynamic, the particular modulation scheme used inthe transmission is not known at the receiver. Also, an LDPC code, as ablock code, can only be decoded if the coded frame is clearlyidentified; i.e., the starting and ending point of the frame has to bedetermined before decoding. Consequently, any framing informationinserted into the transmission stream will not be able to be protectedby the powerful LDPC coding scheme. Furthermore, due to thepower-efficiency of LDPC, the system 100 can operate at extremely lowSNRs; for instance, for BPSK rate ½, the LDPC requires only −2 dB Es/No.Given the many possibilities of LDPC codes and modulation schemes, theframing information needs to identify which particular coding andmodulation schemes are used for the LDPC coded frame that follows theframing information.

[0047] Therefore, it is recognized that that framing information has tobe properly embedded to be recoverable at such low signal to noise ratiowithout the benefit of the LDPC decoder 307. The framing information, asevident from the above discussion, has to efficiently conveyinformation, such as modulation, coding, and pilot structure, beyondmerely the start and end of the frame.

[0048] As seen in FIG. 4, the framing structure 400 includes a UniqueWord (UW) 401, and a physical layer signaling information field 403 thatis denoted a MODulation CODE (MODCODE) field. The UW 401 contains a bitpattern that assists with frame synchronization. The UW 401 is fixed andknown to the receiver. The MODCODE field 403, in an exemplaryembodiment, is a Forward Error Correction (FEC) coded block (e.g.,Reed-Muller coding) and conveys the necessary information for thedemodulator 305 and the LDPC decoder 307 to function properly to decodethe receive signals. For example, the MODCODE field 403 specifiesframing information including the rate of the LDPC codes, the modulationscheme as well other information such as the length of the LDPC codesand the pilot configurations. The modulation scheme supported by theframe 400 can include BPSK, QPSK, 8PSK, 16-ary, 32-ary modulation.Because of the information it provides, the MODECODE field is alsotermed as “physical layer signaling” field.

[0049] Clearly, any information that is to be sent through a noisychannel (rather than pure protocol) needs to be protected properly.Accordingly, following the MODCODE field 403 is a LDPC coded frame 405.To support broadcasting and interactive services, the length of the LDPCcoded frame 405 can be to 64800 bits, and the combined length of the UW401 and the MODCODE field 403 is 90 bits.

[0050] Conventional wisdom has been that because the MODCODE field 401 bvaries with the information being carried, such physical layer signalinginformation field 403 cannot be used for frame detection. According toone embodiment of the present invention, a mechanism is provided toembed a structure within the MODCODE field 403 that can be easilyleveraged for detection purposes, without compromising the errorcorrection capability of the MODCODE field 403.

[0051] The frame structure 400 advantageously requires low overhead,while providing reliable acquisition. The acquisition scheme for rapidacquisition is more fully described below with respect to FIGS. 7 and11. It is noted that rapid acquisition is critical for digital videobroadcasting applications, in that the viewing experience is affectedwhen a viewer switches from channel to channel.

[0052] Therefore, the MODCODE 403 is generated to protect the framinginformation in such a way that the embedded structure in the framingcode can also be utilized for detection and acquisition purpose, as nextdiscussed.

[0053]FIG. 5 is a diagram of a physical layer signaling informationfield generator utilizing a Binary Phase Shift Keying (BPSK)constellation, in accordance with an embodiment of the presentinvention. From the perspective of frame synchronization, the UW 401 atthe beginning of the frame 400 is known and can constitute any sequencewith good correlation property. Accordingly, the present inventionconcentrates on the generation of the physical layer signalinginformation field 403. In this example, the MODCODE generator 500 canreside in the framing module 204 of the transmitter 204. The generator500 includes a Reed-Muller (RM) encoder 501 that outputs a bit stream toa BPSK Constellation Mapper 503. The operation of this generator 500 isnow described, with respect to FIG. 6.

[0054]FIG. 6 shows a flowchart of the operation of a physical layersignaling information field generator, in accordance with an embodimentof the present invention. As previously mentioned, the MODCODE 403conveys information regarding, for example, modulation, FEC code rate,frame length, and configuration of pilot (e.g., whether there is a pilotpresent). Conceptually, the generator 500 outputs a MODCODE 403 that isan interleaving of a block code and its scrambled version.

[0055] In particular, per step 601, a block code is generated, forinstance, using the Reed-Muller encoder 501 to create a code [32,6,16]to carry 6 bits of information. An exemplary generator matrix is givenas follows: $\begin{matrix}0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 \\0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 \\0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\end{matrix}$

[0056] Next, the encoded data is then mapped into BPSK modulation viathe BPSK Constellation Mapper 503, as in step 603. It is contemplatedthat other signal constellations corresponding to different modulationschemes can be utilized; e.g., QPSK. This mapping to a BPSK signalconstellation regardless of the modulation scheme of the user data. Instep 605, the resultant 32 BPSK symbols are duplicated and demultiplexedinto two coded data blocks. It is noted that an additional bit ofinformation can be carried by multiplying, per step 607, the unscrambledcoded data block with {a,−a} through a multiplier 505, whereby a can beany constant, and the signs (i.e., positive and negative) of theconstant represent logical 0 and logical 1, respectively. It is notedthat the sign does not change over the entire block of the duplicated 32bits.

[0057] In step 609, the two data streams are multiplexed back by amultiplexer 507 into one data stream to generate the MODCODE 403 of 64complex symbols (step 611 ). This effectively interleaves the two datastream. It is noted that this fundamentally differs from merelyrepeating the coded symbol, which results in an inferior errorcorrection code—i.e., a linear code of parameters [64,6,32], which canbe readily verified not to be optimal. By contrast, the MODCODE 403output from the generator 500 is equivalent to a permutated [64,7,32]first order Reed-Muller code, which is an optimal code for the givendimension and information rate. Therefore, the error correctioncapability and the data rate are not compromised. One advantage with thepermutated first-order Reed-Muller code is that such a code can bedecoded by the well known fast Hadamard transform in a maximallikelihood way. Moreover, the MODCODE 403 can be used for acceleratingthe acquisition of the frame.

[0058] In step 613, the MODCODE 403 is scrambled via a scrambler 509using, for instance, the following binary sequence: $\begin{matrix}0 & 1 & 1 & 1 & 0 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 1 & 0 & 0 & 1 \\0 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 1 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 0 & 1 & 0\end{matrix}.$

[0059] This scrambling sequence improves the spectral/correlationproperty of the interleaved first-order Reed-Muller code. This improvedcorrelation property is critical for detection and acquisition.

[0060] From the above discussion, it is observed that if the MODCODE 403is parsed into 32 pairs of adjacent symbols, the differential of eachpair is known up to a scaling constant 1 or −1. This property permitsuse of the MODCODE 403 for acquisition purposes when the framinginformation that the MODCODE 403 conveys is unknown. Equivalently, theMODCODE 403 can be described entirely in the binary domain rather thanin the modulated domain. In this case, the output of the RM[32,6,16]encoder 501 can be denoted as (y₁y₂ . . . y₃₂) If the additional bit tobe transmitted equals logical 0, then the output before the scrambler is(y₁y₁y₂y₂ . . . y₃₂y₃₂); i.e., each bit is repeated. Whereas if theadditional bit to be transmitted equals logical 1, the output before thescrambler 509 is equal to (y₁{overscore (y)}₁y₂{overscore (y)}₂ . . .y₃₂{overscore (y)}₃₂); i.e., the repeated symbol is further binarycomplemented. The binary scrambled sequence can be mapped into anypredetermined modulation scheme, such as BPSK, QPSK etc.

[0061] For other system consideration, it is possible to modulate theoriginal and the repeated symbols (or the repeated and binarycomplemented) in a different format. For instance, according to analternative embodiment of the present invention, the original and therepeated symbols are both modulated as BPSK; however, the repeatedsymbol (or repeated and binary complemented) can be rotated by 90degree. In this way, peak to average ratio can be reduced to improve theefficiency of the power amplifier (not shown) of the transmitter 200.

[0062]FIG. 7 is a flowchart of a frame detection process, in accordancewith an embodiment of the present invention. The detection process isexplained with respect to the detector 800 shown in FIG. 8. Detectioninvolves locating the Unique Word 401 and the MODCODE field 403. Thisprocess can accommodate a relatively large frequency offset (e.g., 10-20percent of the symbol rate) by employing differential detection. It isassumed, in the system of FIG. 8, that there is one sample per symbol.The process can readily be adapted to multiple samples per symbol, asrecognized by those skilled in the art. Two scenarios are considered inthe detection process: when the framing information conveyed by theMODCODE is unknown, and when the framing information is known.

[0063] The operation of the detector 800 and detector 900 (of FIG. 9)respectively take into account of the unknown and known scenarios. Asseen in FIG. 8, the incoming signal is first differentiated, as in step701, by shifting the signal into the shift register 801. That is, theincoming symbol is multiplied via a multiplier 803 by the conjugate ofone symbol period delayed signal. The output is then buffered by theshift register 801. Assuming the generator 500 of FIG. 5 is employed,the contents of the rightmost, for example, 25 stages (or cells) of theshift register 801 are multiplied using multipliers, 807 with conjugateof the differentiated unique word, per step 703.

[0064] The values of the multipliers 805, 807 can be readily derived andchecked as follows. The shift register 801 is initialized with allzeros. The unique word 401 and the MODCODE codeword 403 (including theeffect of the scrambling sequence and/or the effect of the potentialrelative rotation of different modulation scheme for the repeated (andcomplemented) symbol) are fed into this detection circuit 800, such thatwhen the content of the rightmost cell of the shift register 801 becomesnonzero for the first time, the conjugates of the contents of the cellsyield the respective values of the multipliers 805, 807 that theparticular cell is connected to. Clearly, these multipliers 805, 807 arefully determined by the way of unique word, MODCODE, scramblingsequence, and the modulation scheme of the physical layer signalingfield and can be derived offline. The outputs of the multipliers 805,807 are summed together, per step 705, through summers 809, 811. In thisexample, only 32 of the leftmost 64 cells of the shift register 801 areused at any given time. These 32 cells are even spaced and indexed fromleft to right as cell number 1,3, . . . 63.

[0065] The outputs of the two summers 809, 811 are respectively added byadder 813 and subtracted by subtractor 815, as in step 707, to producetwo inputs for circuitry 817, which determines the maximum of theabsolute values of the two inputs (step 709). This maximum is thenoutput to a peak search detector 819, per step 711. The operation of thepeak search detector 819 is more fully described below with respect toFIGS. 10 and 11.

[0066] The above process of FIG. 7 and associated detector 800 addressthe case in which the information of the MODCODE is unknown. Thedetector 800 can be streamlined if the MODCODE information is known, asshown in FIG. 9

[0067]FIG. 9 is a diagram of a differential detector, in accordance withan embodiment of the present invention. The detection scheme supportedby a detector 900 when the information carried by the MODCODE is knownbefore acquisition. This information can be made known, for example, byestablishing a dedicated channel to transport the configurationinformation to the receivers; this arrangement is particularly germaneto the broadcast system 100. Upon cold start, the receiver (i.e.,digital modem 105 ) can tune into this predefined channel to receive theconfiguration information. In this case, the information carried by theMODCODE can be inferred from the configuration information, such thatthe acquisition strategy of the detector 900 can be readily deployed.

[0068] As with the detector 800 of FIG. 8, the incoming signal ismultiplied using a multiplier 901 with the conjugate of the signal. Thedifference between the detector 800 and the detector 900 is that thesummation is over the shift register 901 after being multiplied by thecorresponding differentiated UW 401 and the MODCODE 403. As with thedetector 800, the values of the multipliers 905 are determined byinitializing the shift register 903 with all zeros and feeding theunique word 401 and the MODCODE 403 into this circuit 900, when thecontent of the rightmost cell of the shift register becomes nonzero forthe first time, the conjugates of the contents of the cells give therespective values of the multipliers 905 that the particular cell isconnected to. The outputs of all the multipliers 905 are fed to a commonsummer 907.

[0069]FIG. 10 is a diagram of a peak search detection scheme, inaccordance with an embodiment of the present invention. The peak searchdetector 819 of FIG. 8 essentially searches for a peak value within asearch window 1001, and designates this peak value as a candidate bystoring the information is a buffer 1003 as, for example, Candidate 1.This search can be conducted over multiple search windows 1001,resulting in other candidates (e.g., Candidate 2 and Candidate 3). Aftereach search, the candidate is verified by deriving the location of thenext peak from the particular candidate. If the prediction is correct,an acquisition is declared.

[0070] This above process advantageously provides rapid acquisition overthe conventional peak search process. The conventional peak searchprocess sets up a threshold once there is one correlation that is abovethe threshold. In such a case, a candidate is acquired. Thereafter, theprocess verifies whether it is a valid unique word. This conventionalapproach is slow because the thresholding can yield numerous candidates,whereby the verification process is executed for each candidate.

[0071] The details of the peak search process, according to anembodiment of the present invention, are shown in FIG. 11. The design ofthe peak search process stems from the recognition the system 100 mayuse different code rates and different modulation schemes (e.g., BPSK,QPSK, 8 PSK, 16 APSK, and etc.). Even though the modulation scheme maynot be known in advance, the maximal distance between unique words 401is known. The peak search process exploits this knowledge, as nextexplained.

[0072] In step 1101, the process determines whether the modulationscheme is known. Such information can be used to define the searchwindow length (L). For example, if the code length of the LDPC is fixedat 64800 bits, for BPSK, the distance between two unique words is 64800bits. For QPSK, the length is 32400 bits, and for 8 PSK, the length is21600 bits. Thus, for the contemplated modulation schemes, the maximumsize would be based on the length of the LDPC frame. Accordingly, thesearch window, L, can be set as the length of the LDPC frame plus thelength of UW 401 and the MODCODE 403 (e.g., 64800+90) in the case thatmodulation scheme is not known before hand, per step 1103. However, ifthe modulation scheme is known, the search window is set to match thelength of the frame for the particular modulation scheme, as in step1105. The peak search detector 819, per step 1107, finds a peak with thespecified the search window. The search is conducted for the peak withinthis window even though there can be multiple unique word 401 andMODCODE 403 with a search window. The manner in which the search windowis set guarantees that at least one unique word 401 exists is within thesearch window (as shown in FIG. 10).

[0073] Next, the peak location within the search window is designated asa candidate, per step 1109. For each candidate, the MODCODE 403 isdecoded if the modulation and coding information is not yet available(per step 1111). Based on the modulation and coding scheme, next uniqueword location is derived, as in step 1113. Thereafter, the processverifies, per step 1115, whether the predicated location is indeed theUW 401 and the MODCODE 403. If the next consecutive predicted locations(e.g., two) are verified as the UW 401 and the MODCODE 403, then theprocess declares that frame synchronization is acquired.

[0074] The above process can be performed serially or in parallel withrespect to the candidates until one of them is successfully verified.

[0075]FIG. 12 is a diagram of the detector of FIG. 8 modified forbuffering and accumulation, in accordance with an embodiment of thepresent invention. The detector 800 can be modified to incorporate amemory 1201 and an accumulator 1203. After the first differentialmultiplier 803, the data of length L is buffered in the memory 1201, andthe next block of data of L is summed by the accumulator 1203 togetherwith the buffered data. This modification improves the acquisition speedof the detector 800. In a LDPC system, the LDPC decoder 307 has readilyavailable memory for the decoding process; thus, such memory can beutilized for the buffering of the detector 800. That is, the memory 1201can be shared with the decoder 307, thereby avoiding additional cost.

[0076]FIG. 13 illustrates a computer system upon which an embodimentaccording to the present invention can be implemented. The computersystem 1300 includes a bus 1301 or other communication mechanism forcommunicating information, and a processor 1303 coupled to the bus 1301for processing information. The computer system 1300 also includes mainmemory 1305, such as a random access memory (RAM) or other dynamicstorage device, coupled to the bus 1301 for storing information andinstructions to be executed by the processor 1303. Main memory 1305 canalso be used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by theprocessor 1303. The computer system 1300 further includes a read onlymemory (ROM) 1307 or other static storage device coupled to the bus 1301for storing static information and instructions for the processor 1303.A storage device 1309, such as a magnetic disk or optical disk, isadditionally coupled to the bus 1301 for storing information andinstructions.

[0077] The computer system 1300 may be coupled via the bus 1301 to adisplay 1311, such as a cathode ray tube (CRT), liquid crystal display,active matrix display, or plasma display, for displaying information toa computer user. An input device 1313, such as a keyboard includingalphanumeric and other keys, is coupled to the bus 1301 forcommunicating information and command selections to the processor 1303.Another type of user input device is cursor control 1315, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to the processor 1303 and forcontrolling cursor movement on the display 1311.

[0078] According to one embodiment of the invention, the various framesynchronization processes can be provided by the computer system 1300 inresponse to the processor 1303 executing an arrangement of instructionscontained in main memory 1305. Such instructions can be read into mainmemory 1305 from another computer-readable medium, such as the storagedevice 1309. Execution of the arrangement of instructions contained inmain memory 1305 causes the processor 1303 to perform the process stepsdescribed herein. One or more processors in a multi-processingarrangement may also be employed to execute the instructions containedin main memory 1305. In alternative embodiments, hard-wired module maybe used in place of or in combination with software instructions toimplement the embodiment of the present invention. Thus, embodiments ofthe present invention are not limited to any specific combination ofhardware module and software.

[0079] The computer system 1300 also includes a communication interface1317 coupled to bus 1301. The communication interface 1317 provides atwo-way data communication coupling to a network link 1319 connected toa local network 1321. For example, the communication interface 1317 maybe a digital subscriber line (DSL) card or modem, an integrated servicesdigital network (ISDN) card, a cable modem, or a telephone modem toprovide a data communication connection to a corresponding type oftelephone line. As another example, communication interface 1317 may bea local area network (LAN) card (e.g. for Ethernet™ or an AsynchronousTransfer Model (ATM) network) to provide a data communication connectionto a compatible LAN. Wireless links can also be implemented. In any suchimplementation, communication interface 1317 sends and receiveselectrical, electromagnetic, or optical signals that carry digital datastreams representing various types of information. Further, thecommunication interface 1317 can include peripheral interface devices,such as a Universal Serial Bus (USB) interface, a PCMCIA (PersonalComputer Memory Card International Association) interface, etc.

[0080] The network link 1319 typically provides data communicationthrough one or more networks to other data devices. For example, thenetwork link 1319 may provide a connection through local network 1321 toa host computer 1323, which has connectivity to a network 1325 (e.g. awide area network (WAN) or the global packet data communication networknow commonly referred to as the “Internet”) or to data equipmentoperated by service provider. The local network 1321 and network 1325both use electrical, electromagnetic, or optical signals to conveyinformation and instructions. The signals through the various networksand the signals on network link 1319 and through communication interface1317, which communicate digital data with computer system 1300, areexemplary forms of carrier waves bearing the information andinstructions.

[0081] The computer system 1300 can send messages and receive data,including program code, through the network(s), network link 1319, andcommunication interface 1317. In the Internet example, a server (notshown) might transmit requested code belonging to an application programfor implementing an embodiment of the present invention through thenetwork 1325, local network 1321 and communication interface 1317. Theprocessor 1303 may execute the transmitted code while being receivedand/or store the code in storage device 139, or other non-volatilestorage for later execution. In this manner, computer system 1300 mayobtain application code in the form of a carrier wave.

[0082] The term “computer-readable medium” as used herein refers to anymedium that participates in providing instructions to the processor 1303for execution. Such a medium may take many forms, including but notlimited to non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 1309. Volatile media include dynamic memory, such asmain memory 1305. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 1301.Transmission media can also take the form of acoustic, optical, orelectromagnetic waves, such as those generated during radio frequency(RF) and infrared (IR) data communications. Common forms ofcomputer-readable media include, for example, a floppy disk, a flexibledisk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM,CDRW, DVD, any other optical medium, punch cards, paper tape, opticalmark sheets, any other physical medium with patterns of holes or otheroptically recognizable indicia, a RAM, a PROM, and EPROM, a FLASH-EPROM,any other memory chip or cartridge, a carrier wave, or any other mediumfrom which a computer can read.

[0083] Various forms of computer-readable media may be involved inproviding instructions to a processor for execution. For example, theinstructions for carrying out at least part of the present invention mayinitially be borne on a magnetic disk of a remote computer. In such ascenario, the remote computer loads the instructions into main memoryand sends the instructions over a telephone line using a modem. A modemof a local computer system receives the data on the telephone line anduses an infrared transmitter to convert the data to an infrared signaland transmit the infrared signal to a portable computing device, such asa personal digital assistance (PDA) and a laptop. An infrared detectoron the portable computing device receives the information andinstructions borne by the infrared signal and places the data on a bus.The bus conveys the data to main memory, from which a processorretrieves and executes the instructions. The instructions received bymain memory may optionally be stored on storage device either before orafter execution by processor.

[0084] Accordingly, the various embodiments of the present inventionprovide an approach for achieving frame synchronization in a digitalbroadcast system utilizing Low Density Parity Check (LDPC) codes. Aframing module includes a constellation mapper for mapping a codeword(e.g., generated by a Reed-Muller encoder) specifying framinginformation of a frame according to a signal constellation to output adata stream. The data stream is split into two data streams. One of thedata stream is modified to interleave additional bits (each of which iseither a duplicate bit or a binary complement bit). The two data streamsare then combined to form the physical layer header, which is appendedto an LDPC coded frame. This approach embeds a framing structure thatcan assist with synchronization. On the receiving side, a relativelysimple frame detector can be used to locate the unique word and physicallayer header based on the embedded framing structure of the physicallayer header. This information is then supplied to a peak searchdetection process, which searches for a peak value within a searchwindow, and designates this peak value as a candidate. The search windowlength can be set according to the modulation scheme employed, if known;otherwise, a default length is used. The peak search can be conductedover multiple search windows, resulting in other candidates. After eachsearch, the candidate is verified by deriving the location of the nextpeak from the particular candidate. The above arrangement advantageouslyprovides rapid and reliable frame acquisition without additionaloverhead.

[0085] While the present invention has been described in connection witha number of embodiments and implementations, the present invention isnot so limited but covers various obvious modifications and equivalentarrangements, which fall within the purview of the appended claims.

What is claimed is:
 1. A method for supporting frame synchronization ina digital communication system, the method comprising the steps of:mapping a codeword specifying framing information of a frame accordingto a signal constellation to output a data stream; duplicating anddemultiplexing the data stream into a first data stream and a seconddata stream; modifying the first data stream according to apredetermined operation; multiplexing the modified first data streamwith the second data stream; and outputting a physical layer signalingheader corresponding to the frame based on the multiplexed data streams.2. A method according to claim 1, wherein the signal constellation isindependent of a modulation scheme of the frame.
 3. A method accordingto claim 1, wherein the frame is a Low Density Parity Check (LDPC) codedframe.
 4. A method according to claim 1, wherein the predeterminedoperation includes multiplying the first data stream with {−a,} or {a},a being a predetermined constant.
 5. A method according to claim 4,wherein the sign of the multiplier represents a portion of the framinginformation.
 6. A method according to claim 1, wherein themultiplication results in bits of the first data stream interleaved withrespective additional bits, the additional bits being phase rotatedrelative to the bits of the first data stream during modulation.
 7. Amethod according to claim 1, further comprising the step of: generatingthe codeword according to a first order Reed-Muller code.
 8. A methodaccording to claim 1, wherein the framing information specifies amodulation scheme, and a coding scheme.
 9. A method according to claim1, further comprising the step of: scrambling the multiplexed datastreams.
 10. A method according to claim 1, wherein the signalconstellation is according to a Binary Phase Shift Keying (BPSK) scheme.11. A computer-readable medium bearing instructions for supporting framesynchronization in a digital communication system, said instruction,being arranged, upon execution, to cause one or more processors toperform the method of claim
 1. 12. An apparatus for supporting framesynchronization in a digital communication system, the apparatuscomprising: a constellation mapper configured to map a codewordspecifying framing information of a frame according to a signalconstellation to output a data stream, wherein the data stream isdemultiplexed into a first data stream and a second data stream; amultiplier coupled to the constellation mapper and configured to modifythe first data stream; and a multiplexer configured to combine themodified first data stream with the second data stream, wherein aphysical layer signaling header corresponding to the frame is outputbased the multiplexed data streams.
 13. An apparatus according to claim12, wherein the signal constellation is independent of a modulationscheme of the frame.
 14. An apparatus according to claim 12, wherein theframe is a Low Density Parity Check (LDPC) frame.
 15. An apparatusaccording to claim 12, wherein the multiplier multiplies the first datastream with {−a,} or {a}, a being a predetermined constant.
 16. A methodaccording to claim 15, wherein the sign of the multiplier represents aportion of the framing information.
 17. An apparatus according to claim12, wherein the multiplication results in bits of the first data streaminterleaved with respective additional bits, the additional bits beingphase rotated relative to the bits of the first data stream duringmodulation.
 18. An apparatus according to claim 12, further comprising:a code generator coupled to the constellation mapper and configured togenerate the codeword according to a first order Reed-Muller code. 19.An apparatus according to claim 12, wherein the framing informationspecifies a modulation scheme, and a coding scheme.
 20. An apparatusaccording to claim 12, further comprising: a scrambler configured toscramble the multiplexed data streams.
 21. An apparatus according toclaim 12, wherein the signal constellation is according to a BinaryPhase Shift Keying (BPSK) scheme.
 22. A method of supporting framesynchronization in a digital broadcast system, the method comprising thesteps of: encoding framing information of a frame by a forward errorcorrection code to output encoded bits; repeating each of the encodedbits; and modifying the repeated bits according to a predeterminedoperation to transmit additional framing information.
 23. A methodaccording to claim 22, wherein the predetermined operation includescomplementing the repeated bits.
 24. A method according to claim 22,wherein the forward error correction code is a first order Reed-Mullercode.
 25. A method according to claim 22, wherein the frame is a LowDensity Parity Check (LDPC) coded frame.
 26. A computer-readable mediumbearing instructions for supporting frame synchronization in a digitalbroadcast system, said instruction, being arranged, upon execution, tocause one or more processors to perform the method of claim
 22. 27. Amethod for detecting the start of a frame, the method comprising thesteps of: receiving a data stream corresponding to a broadcast signal,the data stream including a unique word and a physical layer signalingheader specifying modulation and coding information of the broadcastsignal; differentiating the received data stream; multiplying thedifferentiated data stream with a predetermined multiplier; summingoutputs of the multiplication; adding the summed outputs to yield aplurality of added values; subtracting the summed outputs to yield aplurality of subtracted values; and determining a maximum value amongabsolute values of the added values and the subtracted values.
 28. Amethod according to claim 27, wherein the broadcast signal includes aLow Density Parity Check (LDPC) coded frame.
 29. A method according toclaim 27, further comprising the step of: receiving the broadcast signalover a satellite communication channel.
 30. A method according to claim27, further comprising the step of: outputting the maximum value to adetector configured to determine location of the unique word.
 31. Acomputer-readable medium bearing instructions for detecting the start ofa frame, said instruction, being arranged, upon execution, to cause oneor more processors to perform the method of claim
 27. 32. A device fordetecting the start of a frame, the device comprising: means forreceiving a data stream corresponding to a broadcast signal, the datastream including a unique word and a physical layer signaling headerspecifying modulation and coding information of the broadcast signal;means for differentiating the received data stream; means formultiplying the differentiated data stream with a predeterminedmultiplier; means for summing outputs of the multiplication; means foradding the summed outputs to yield a plurality of added values; meansfor subtracting the summed outputs to yield a plurality of subtractedvalues; and means for determining a maximum value among absolute valuesof the added values and the subtracted values.
 33. A device according toclaim 32, wherein the broadcast signal includes a Low Density ParityCheck (LDPC) coded frame.
 34. A device according to claim 32, furthercomprising: means for receiving the broadcast signal over a satellitecommunication channel.
 35. A device according to claim 32, furthercomprising: means for outputting the maximum value to a detectorconfigured to determine location of the unique word.
 36. A method forrecovering framing information of a frame transmitted over in a digitalcommunication system, the method comprising the steps of: descrambling aphysical layer signaling code of the frame, the physical layer signalcode being encoded according to a first order Reed-Muller code andinterleaved; and decoding the physical layer signaling code to derivecoding rate, modulation format, and pilot structure of the frame.
 37. Amethod according to claim 36, further comprising the step of:deinterleaving the physical layer signaling code prior to the decodingstep.
 38. A method according to claim 36, wherein the decoding of thephysical layer signaling code employs a fast Hadamard transform.
 39. Acomputer-readable medium bearing instructions for recovering framinginformation of a frame transmitted over in a digital communicationsystem, said instruction, being arranged, upon execution, to cause oneor more processors to perform the method of claim
 36. 40. A method forsupporting frame synchronization in a digital communication system, themethod comprising the steps of: setting a search window length;determining location of a peak within a frame over the search windowlength, the frame including a unique word, a codeword, and a codedsegment, wherein the codeword specifies framing information of the codedsegment; designating the peak location as a candidate; verifying thecandidate; and declaring acquisition of the frame if the candidate isverified.
 41. A method according to claim 40, further comprising thesteps of: decoding the codeword based on the candidate; and predictinglocation of a next peak.
 42. A method according to claim 40, wherein theframing codeword specifies a modulation scheme and a coding scheme ofthe frame.
 43. A method according to claim 40, wherein the frame is aLow Density Parity Check (LDPC) frame.
 44. A method according to claim40, further comprising the steps of: determining a modulation schemeassociated with the frame; setting the search window length according tothe determined modulation scheme; and if the modulation scheme cannot bedetermined, setting the search window length to a default value based onthe length of the frame.
 45. A method according to claim 40, furthercomprising the step of: iteratively conducting subsequent peak searcheswith respect to other frames according to the set search window to yielda plurality of candidates, wherein the acquisition is declared after apredetermined number of candidates are successfully verified.
 46. Amethod according to claim 40, wherein the peak corresponds to the uniqueword within the frame.
 47. A computer-readable medium bearinginstructions for supporting frame synchronization in a digital broadcastsystem, said instruction, being arranged, upon execution, to cause oneor more processors to perform the method of claim
 40. 48. A transmittercomprising: an encoder configured to output a Low Density Parity Check(LDPC) codeword; and a framing module configured to generate a LDPCcoded frame in response to the LDPC codeword, and to append a physicallayer signaling field to the LDPC codeword for specifying modulation andcoding information associated with the LDPC coded frame, wherein thephysical layer signaling field is encoded with a Forward ErrorCorrection (FEC) code and has an embedded framing structure to assistwith frame synchronization.
 49. A transmitter according to claim 48,further comprising: means for broadcasting a signal over a satellitecommunication channel, the signal representing the physical layersignaling field and the LDPC coded frame.